Method for directing groundwater flow and treating groundwater in situ

ABSTRACT

The present invention relates to a method for treating groundwater in situ in rock or soil. An elongate permeable upgradient zone and an elongate permeable downgradient zone, each in hydraulic communication with a permeable subsurface treatment zone and having a major axis parallel to a non-zero component of the general flow direction, are provided in the subsurface by any of a number of construction methods. The upgradient zone, downgradient zone, and treatment zone are situated within the subsurface medium and have permeabilities substantially greater than the adjacent subsurface medium&#39;s permeability. Groundwater is allowed to move from the subsurface medium adjacent to the upgradient zone into the upgradient zone, where the groundwater refracts and moves to a treatment zone. After being treated in the treatment zone by an in situ treatment process, such as a process employing air sparging, sorption or reaction with zero-valent iron, the groundwater moves into, through, and out of the downgradient zone into the subsurface medium adjacent to the downgradient zone. The method does not require pumping. A method for directing groundwater around a particular location to prevent contamination of the groundwater by a contaminant located at the particular location, to prevent migration of a contaminant located at the particular location, to reduce the flow velocity of groundwater in the particular location, or to increase the residence time in an in situ treatment center located downgradient from the particular location is also disclosed.

This application is a division of U.S. Pat. No. 08/681,701, filed Jul.29, 1996, now U.S. Pat. No. 5,833,388.

FIELD OF THE INVENTION

This invention relates to a method for directing the flow of groundwaterand to a method for treating groundwater.

BACKGROUND OF THE INVENTION

Most efforts to address groundwater contamination in subsurface mediahave historically involved pumping to extract contaminated groundwaterfrom the subsurface for above-ground treatment. This process is commonlyreferred to as "pump-and-treat". However, pump-and-treat systems arelimited in their ability to remediate contaminated groundwater, soil,and rock (Mackay et al., Environmental Science and Technology, 23(6) :630-636 (1989) and Travis et al., Environmental Science and Technology,24: 1464-1466 (1990)). Remedial lifetimes are commonly on the order ofdecades to centuries. During pumping, there are ongoing energy and costrequirements, and the need to properly treat the groundwater fordisposal at the surface poses an additional problem.

Conventional pump-and-treat systems are particularly ineffective forcollecting and treating groundwater from low-permeability fracturedrock. Contaminated groundwater in low-permeability fractured rock iscommonly extracted for treatment at the surface by actively pumping fromone or more conventional vertical recovery wells, typically installed ator near the contaminant source for source remediation and at thedowngradient end of a contaminant plume to control contaminantmigration.

Conventional migration control in low-permeability fractured rocktypically requires installation of a sufficient number of recovery wellsto capture all of the contaminated groundwater that would otherwise passby. However, when the fractures in the low-permeability rock areirregularly spaced and poorly interconnected, as they commonly are,large numbers of conventional vertical recovery wells must be placedfairly close together to ensure that most of the contaminatedgroundwater is captured. Sometimes, the range of potential groundwatercapture in low-permeability fractured rock for a single well may be onlyten to thirty feet, especially under unconfined conditions, andattempting to achieve effective capture over a wide region may requireseveral dozen wells equipped with pumps and piping, all of which must beinstalled and maintained at considerable expense. Moreover, in coldclimates, the associated surface lines and other equipment must beprotected from freezing which further adds to the expense. Even withclose well installation, some large fractures or faults may passunnoticed between wells and allow significant migration of contaminantspast the area of pumping.

In recent years, unconventional horizontal wells, rather thanconventional vertical wells, have sometimes been used for groundwaterrecovery, but, in general, these also require active pumping to extractcontaminated groundwater, and they are very expensive to construct.Moreover, horizontal wells frequently fail to capture contaminatedgroundwater in low-permeability fractured rock, because the presence ofcertain extremely low-permeability horizontal rock layers preventsgroundwater from flowing at an appreciable rate in the verticaldirection, except through widely-spaced isolated fractures. Under thehydraulic stress of pumping, there might be negligible or very smallvertical components of flow through these low-permeability layers to thehorizontal-well screens, precluding effective groundwater recovery fromhorizontal rock layers that are separated from the well screen by thelow-permeability layers.

A comparatively new method for recovering groundwater fromlow-permeability fractured rock involves pumping groundwater from alinear blasted-bedrock zone or from a radial set of blasted-bedrockzones (Begor et al., Ground Water, 27: 57-65 (1990); Smith et al.,Proceedings of the 5th Annual Hazardous Materials and EnvironmentalManagement Conference/Central, Chicago, pp. 103-117 (1992); Gehl,Proceedings of the Focus Conference of Eastern Regional GroundwaterIssues, National Water Well Association, pp. 265-273 (1994); and McKownet al., Proceedings of the 21st Annual Conference on Explosives andBlasting Techniques, International Society of Explosives Engineers, pp.305-322 (1995) ("McKown")) The linear zones of shattered rock, commonlyreferred to as "trenches", along with one or more pumping wells, canform an extraction system that effectively connects otherwise naturallyunconnected fractures and greatly increases the effective region ofpumping influence. Since a blasted-bedrock trench has a much greaterhydraulic conductivity than the native rock surrounding it, groundwaterflows in from many directions when the pump in a trench recovery well isoperating. The average flow rate from one or more recovery wellsinstalled in a blasted-bedrock trench exceeds the hypothetical flow rateof more than 60 or 70 traditional recovery wells installed in the sametype of rock (McKown). However, as indicated above, flow into the trenchis effected only when the trench is actively pumped.

One common characteristic of the groundwater "pump-and-treat" methods,irrespective of whether the well employed is a horizontal well, avertical well, or a blasted-bedrock well, is the need to bring thegroundwater collected by the well to the surface for treatment.Generally, this requires more or less continuous pump operation, andthis pumping process, as well as the surface treatment processes, isexpensive in terms of operational and maintenance costs. In addition,effluent from the above-ground treatment of groundwater typically mustbe discharged to a permitted discharge point or to a publicly ownedwastewater treatment facility. Since the treated groundwater isdischarged above ground, chemical substances other than the contaminantswhich necessitated the groundwater treatment must also be dealt with inthe treatment process and be remediated to levels acceptable forabove-ground discharge. For example, when Fe(II) in anaerobicgroundwater is brought to the surface and exposed to air, it commonly isoxidized to Fe(III) which then forms a precipitate, which can causewater-quality problems and foul treatment equipment and lines. Anotherexample of this problem is the need to permit colloidal materials andsilt to settle out the groundwater prior to treatment and/or discharge.Thus, above-ground treatment frequently necessitates the installation ofsystems to treat substances which would never have needed remediationhad the water remained in the subsurface. The long-term costs ofabove-ground effluent treatment and disposal, coupled with the long-termcosts associated with the maintenance and operation of pumps and relatedequipment, makes above-ground treatment of groundwater expensive.

Therefore, a continuing need exists for methods of directing groundwaterflow and treating groundwater. The present invention is directed tomeeting this need.

SUMMARY OF THE INVENTION

The present invention relates to a method for treating groundwaterflowing in a general flow direction through a subsurface medium. Themethod includes providing an elongate permeable upgradient zone and anelongate permeable downgradient zone. The upgradient zone is locatedhydraulically upgradient from and in hydraulic communication with atreatment zone and has a major axis parallel to a non-zero component ofthe general flow direction. The downgradient zone is locatedhydraulically downgradient from and in hydraulic communication with thetreatment zone and has a major axis parallel to a non-zero component ofthe general flow direction. Each of the upgradient zone, downgradientzone, and treatment zone is situated within the subsurface medium andhas a permeability substantially greater than the surrounding subsurfacemedium's permeability. Groundwater is allowed to move from thesubsurface medium surrounding the upgradient zone into and through theupgradient zone to, through, and out of the treatment zone. Thegroundwater is then allowed to move into, through, and out of thedowngradient zone into the subsurface medium surrounding thedowngradient zone. The method further includes treating the groundwaterin the treatment zone.

The present invention also relates to a method for directing groundwaterflowing in a general flow direction through a subsurface medium around aparticular subsurface location. The method includes providing anelongate permeable upgradient zone and an elongate permeabledowngradient zone. The upgradient zone is located hydraulicallyupgradient from the particular location and has a major axis parallel toa non-zero component of the general flow direction. The downgradientzone is located hydraulically downgradient from and in hydrauliccommunication with the upgradient zone, is located downgradient from theparticular location, and has a major axis parallel to a non-zerocomponent of the general flow direction. Each of the upgradient zone anddowngradient zone is situated within the subsurface medium and has apermeability substantially greater than the surrounding subsurfacemedium's permeability. The method further includes allowing groundwaterto move from the subsurface medium surrounding the upgradient zone intoand through the upgradient zone to, through, and out of the downgradientzone into the subsurface medium surrounding the downgradient zone.

The method of the present invention is particularly well suited fordirecting contaminated groundwater without pumping to one or moresubsurface treatment zones in rock or soil where the groundwater can betreated in situ. Because the method permits in situ groundwatertreatment, no pumping is required, and the costs associated with pumpingare avoided. Moreover, the problems associated with discharging effluentfrom above-ground groundwater treatment are avoided. The method of thepresent invention can also be used to direct relatively uncontaminatedgroundwater around a relatively contaminated subsurface location toprevent contamination of the relatively uncontaminated groundwater.Alternatively, the method of the present invention can be used toprotect a relatively uncontaminated subsurface location from becomingcontaminated by a contaminated groundwater flow by directing therelatively contaminated groundwater around the relatively uncontaminatedsubsurface location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a subsurface medium containing an upgradientzone.

FIGS. 2A-2F are plan views of various upgradient and downgradient zoneconfigurations in accordance with the present invention.

FIGS. 3A-3C are plan views of various upgradient and downgradient zoneconfigurations in accordance with the present invention.

FIG. 4 is a schematic illustrating a streamline of groundwater in anupgradient zone.

FIG. 5 is a computer-generated contour plot showing lines of equalhydraulic head in an upgradient and downgradient zone configuration inaccordance with the present invention.

FIG. 6 is a schematic illustrating a streamline of groundwater in anupgradient zone and the transverse spacing of optional additionaltreatment zones in accordance with the present invention.

FIG. 7 is a graph showing the relationship of treatment-zone transversespacing (S), hydraulic conductivity ratio (K₂ /K₁), and upgradient zonethickness (T).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for treating groundwater.Groundwater includes water located below the surface of the ground atgauge pressures greater than zero atmospheres. The groundwater islocated in a subsurface medium. The subsurface medium can be anynaturally occurring or man-made medium, including, for example, soil,rock, and fill. The subsurface medium can include a single type ofmaterial, or it can include two or more types of materials. Moreover,the subsurface medium can have a substantially uniform permeability orcan vary in permeability.

Prior to the installation of the upgradient, downgradient, and treatmentzones (described below), the groundwater flows through the subsurfacemedium along streamlines affected by local permeability variations inthe subsurface medium. Macroscopically, the groundwater has a generalflow direction, which is the weighted average of the microscopic flowsalong all the streamlines in a particular spatial region of thesubsurface medium. It is not critical to the practice of method of thepresent invention that the precise flow direction be known, althoughsuch knowledge would facilitate optimization of the method for aparticular spatial region. The general flow directions can be determinedby well-known techniques, such as those described in Freeze et al.,Groundwater, Englewood Cliffs, N. J. :Prentice-Hall, Inc. (1979) andDomenico et al., Physical and Chemical Hydrogeology, New York: JohnWiley and Sons (1990) ("Domenico"), which are hereby incorporated byreference.

The method includes providing an upgradient zone and a downgradient zonesituated in the subsurface medium in which the groundwater to be treatedis located. The upgradient zone is hydraulically upgradient from and inhydraulic communication with a permeable treatment zone which is alsosituated within the subsurface medium. The downgradient zone ishydraulically downgradient from and in hydraulic communication with thetreatment zone. That is, water in the upgradient zone will, over thecourse of time, flow from the upgradient zone to and through thetreatment zone and from the treatment zone to the downgradient zone.

Each of the upgradient zone, the downgradient zone, and the treatmentzone has a permeability (i.e., an inherent capacity to transmit fluid)substantially greater than the permeability of the subsurface medium. Inparticular, it is preferred that each of these zones has a permeabilitysubstantially greater than the permeability of the subsurface mediumsurrounding the zone. To provide a margin of safety to account forunknown permeability variations in the subsurface medium, it is mostpreferred that the permeabilities of the upgradient, downgradient, andtreatment zones all be substantially greater than the permeability ofthe most permeable portion of the subsurface medium. The absolute valuesof the permeabilities of the upgradient, downgradient, and treatmentzones are not critical to the practice of the present invention. Theimportant factor is that the upgradient, downgradient, and treatmentzones' permeabilities be substantially greater than that of thesubsurface medium. For a relatively low-permeability substrate medium,such as some types of rock or soil, very large permeability contrastsbetween the upgradient, downgradient, and treatment zones, collectively,and the subsurface medium can be readily attained.

The upgradient, downgradient, and treatment zones can be constructed ofpermeable materials, such as porous or fractured media, whosepermeability is enhanced well above the permeability of the surroundingrock, soil, or other subsurface medium, through an engineered process.Suitable porous media include those materials containing open spacesbetween grains, pebbles, or blocks of the solid portion of the media.More particularly, porous media suitable for use as permeable materialsin the practice of the present invention include, but are not limitedto, consolidated materials, such as broken or fractured rock, andunconsolidated materials, such as sand, or gravel.

The compositions and permeabilities of the materials in the upgradient,downgradient, and treatment zones can be the same or different.Generally, for ease of construction, these materials are the same, and,consequently, their permeablilities are also similar. However, in somecases, such as where the geometry of the upgradient and downgradientzones are different or where the permeabilities of the subsurface mediumsurrounding the upgradient and downgradient zones differ, upgradient anddowngradient zones made of materials having different permeabilities maybe desirable.

The permeability of the permeable materials in the upgradient,downgradient, and treatment zones are, preferably, at least two to threeorders of magnitude greater than the permeability of the subsurfacemedium surrounding the upgradient, downgradient, and treatment zones.For example, in the case where the subsurface medium has a hydraulicconductivity on the order of 10⁻⁴ cm/s, permeable materials suitable foruse in the construction of the upgradient, downgradient, and treatmentzones are those having hydraulic conductivities of from about 10⁻³ cm/sto about 10⁻¹ cm/s, preferably from about 10⁻² cm/s to about 10⁻¹ cm/s;where the subsurface medium has a hydraulic conductivity on the order of10⁻⁵ cm/s, suitable permeable materials are those having hydraulicconductivities of from about 10⁻⁴ cm/s to about 10⁻¹ cm/s, preferablyfrom about 10⁻³ cm/s to about 10⁻¹ cm/s; where the subsurface medium hasa hydraulic conductivity on the order of 10⁻⁶ cm/s, suitable permeablematerials are those having hydraulic conductivities of from about 10⁻⁵cm/s to about 10⁻¹ cm/s, preferably from about 10⁻⁴ cm/s to about 10⁻¹cm/s. In addition, it should be noted that the upgradient zone need notnecessarily be of uniform permeability or composition, althoughsubstantial uniformity is generally preferred. Likewise, thedowngradient and treatment zones may each be of uniform or nonuniformpermeability and composition.

The upgradient zone is elongate along a major axis parallel to anon-zero component of the general groundwater flow direction, that is,in a direction which is not perpendicular to the general groundwaterflow. One suitable orientation of the upgradient zone with respect tothe general flow direction of the groundwater is depicted in FIG. 1.Upgradient zone 2, situated in substrate medium 4, is elongate alongmajor axis 6. Major axis 6 and the groundwater's general flow direction8 form an angle, designated α. The angle α can have any value from 0° toup to slightly less than 90°. Preferred orientations of the elongateupgradient zone with respect to the subsurface groundwater flow arethose where angle α is from about 30° to about 60 °. Optimalorientations depend on a variety of factors, such as the permeability ofthe upgradient zone, the permeability of the subsurface medium,particularly the permeability of the subsurface medium surrounding theupgradient zone, and the shape of the upgradient zone. Optimization ofthese variables is discussed in greater detail below.

The downgradient zone is also elongate along a major axis parallel to anon-zero component of the groundwater's general flow direction, that is,in a direction which is not perpendicular to the general groundwaterflow.

The elongate upgradient zone and downgradient zone can have any aspectratio greater than 1. Suitable aspect ratios are, generally, muchgreater that one, usually from about 10 to about 100. Optimal aspectratios are dictated by a number of factors, including orientation of thezones with respect to each other and to the groundwater's general flowdirection, permeability of the upgradient and downgradient zones, andpermeability of the subsurface medium. The upgradient and downgradientzones can be of any elongate shape when viewed in a plane parallelingthe surface of the ground, including, for example, elliptical,rectangular, or triangular. Preferred shapes are those which aresubstantially rectangular, although non-uniform permeabilities along thelength of the upgradient zone or the downgradient zone may cause othershapes to be favored. Preferably, the upgradient and downgradient zonesare of substantially uniform width over a substantial portion of theirlengths. Typically and especially in cases where the permeabilities ofthe subsurface media surrounding the upgradient and downgradient zonesare substantially the same and where the permeabilities of theupgradient and downgradient zones are substantially the same, it ispreferred that the upgradient and downgradient zones be of the sameshape, more preferably rectangular, each having the same, substantiallyuniform width and substantially the same length.

The upgradient and downgradient zones can be oriented in a number ofspatial configurations in the practice of the present invention. Severalsuitable orientations are shown in FIGS. 2A, 2B, and 2C. Flowlines 10show the path groundwater having general flow direction 8 takes throughsubsurface medium 4 under the influence of upgradient zone 2,downgradient zone 12, and treatment zone 14 when upgradient zone 2 iselongate along major axis 16 and downgradient zone 12 is elongate alongmajor axis 18. In one suitable orientation, depicted in FIG. 2A, themajor axes of the upgradient and downgradient zones are coincident, andthe upgradient and downgradient zones form a line oriented in anydirection other than perpendicular to the general flow direction of thegroundwater. In one particular illustration of this orientation,depicted in FIG. 2B and designated the "I-shape", the upgradient anddowngradient zones' major axes are coincident with each other andparallel to the groundwater's general flow direction. Anotheradvantageous shape is that of a "sideways-V", such as depicted in FIG.2C. The sideways-V has an apex at the treatment zone and legs,representing the upgradient and downgradient zones, pointing away from aline parallel to the general flow direction. As indicated before, theupgradient zone's major axis forms an angle, α, with the flow direction.In the sideways-V orientation, the downgradient zone is preferablyelongated along an axis which forms an angle equal to 180-α, asindicated in FIG. 2C. When configured in this way, the angle between themajor axes of the upgradient and downgradient zones is 180-2α.

As indicated above, the upgradient: and downgradient zones are inhydraulic communication with the treatment zone. For purposes of thisapplication, two zones have the quality of being "in hydrauliccommunication" when the zones abut one another or are separated by someintervening material which does not significantly reduce the movement offluid from one zone to the other. Preferably, the intervening materialhas a permeability at least equal to the permeability of one of the twozones in hydraulic communication. However, intervening materials ofsomewhat lower permeability can be acceptable.

Having provided the upgradient and downgradient zones, groundwater isallowed to enter the upgradient zone and move through the upgradientzone to the treatment zone. The groundwater is then allowed to move fromthe treatment zone to the downgradient zone, through the downgradientzone, and out of the downgradient zone into the subsurface mediumsurrounding the downgradient zone. Groundwater movement into, through,and out of the upgradient, downgradient, and treatment zones occurswithout pumping, governed by the forces created by the existence of theupgradient, downgradient, and treatment zones of higher permeabilitythan the permeability of the subsurface medium. Subsurface groundwaterflowing in a general flow direction contacts the subsurfacemedium-upgradient zone interface and moves into the zone. Subsurfacegroundwater water within the upgradient zone then moves under hydraulicforces within and through the upgradient zone for some distance in adirection semi-parallel to the upgradient zone's major axis to and intothe treatment zone. The water then is directed by hydraulic forces outof the treatment zone, into the downgradient zone, through at least aportion of the downgradient zone, and out of the downgradient zone intothe subsurface medium surrounding the downgradient zone. Optimally, theshape and orientation of the upgradient and downgradient zones are suchthat the rate of movement of groundwater out of the downgradient zone isthe same as the rate of movement of subsurface groundwater into theupgradient zone, thereby avoiding hydraulic head buildup within theupgradient and downgradient zones.

The method of the present invention can also be practiced using aplurality of upgradient and downgradient zones. In this embodiment themethod further includes providing one or more additional elongateupgradient zones and one or more additional elongate downgradient zones.Each additional upgradient and downgradient zone has a permeabilitywhich is substantially greater that the permeability of the subsurfacemedium, and each has a major axis parallel to a non-zero component ofthe flow direction. Preferably, the number of additional of upgradientzones is equal to the number of additional downgradient zones so thatthe total number of upgradient zones is equal to the total number ofdowngradient zones. In a particularly preferred embodiment, eachdowngradient zone is elongate along a major axis which is coincidentwith the major axis of one of the upgradient zones, so that eachdowngradient zone has exactly one corresponding upgradient zone alongwhose major axis it is elongate. The orientation of the additionalupgradient and downgradient zones is not critical to the practice of thepresent invention. However, it is preferred that the downgradient zoneand the additional downgradient zones be mirror images of the upgradientzone and the additional upgradient zones reflected in a planeperpendicular to general flow direction and passing through thetreatment zone. Optimally, the total number of upgradient zones is 2,and the total number of downgradient zones is 2. In this configurationthe upgradient and downgradient zones form two sideways V's having acommon apex, the apex representing the treatment zone to whichgroundwater is directed. This configuration, designated theX-configuration because of the shape defined by the two upgradient andtwo downgradient zones when viewed in a plane parallel to the surface ofthe ground, is depicted in FIG. 2D. As indicated before, the preferredconfiguration of each sideways V making up the X is such that the anglebetween the general flow direction and the downgradient zone's majoraxis is 180-α, where a is the angle between the upgradient zone's majoraxis and the general flow direction. In a more preferred configuration,the downgradient zone and the additional downgradient zone are mirrorimages of the upgradient zone and the additional upgradient zonereflected in a plane perpendicular to the ground surface, parallel tothe general flow direction, and passing through the treatment zone.

As the skilled artworker will note, additional configurations arepossible including, for example, multiple, laterally adjacentconfigurations. The method of the present invention can be used todirect groundwater to a series of treatment zones by employing a seriesof paired upgradient and downgradient zones with each pair of zoneshydraulically connected to a single treatment zone. One example of sucha configuration is a series of laterally adjacent sideways V's whereeach V in the series points in the same direction. Another example ofsuch a configuration is a series of laterally adjacent X's, asillustrated in FIG. 2E. Using multiple treatment zones, a broader areaof groundwater flow can be directed to the series of treatment zones fortreatment. Other combinations of sideways V's and X's, and I's can beemployed, including those where the downgradient zone of one treatmentcenter is used as the upgradient zone of a second treatment center, asdepicted, for example, in FIG. 2F. Optionally, additional treatmentzones can be located at various locations along the upgradient ordowngradient zones.

The upgradient and downgradient zones can be formed by any suitablemethod, many of which are known to those skilled in the art.

The preferred method for installation in soil (i.e., when the subsurfacemedium is soil), in most instances, is that of removal of the soil fromthe upgradient or downgradient zone, such as by excavation, and fillingthe trenches with permeable materials, such as sand, gravel, brokenrock, or other high-permeability porous media. Where gravel is employed,it is preferably used in conjunction with a protective geotextile fabricor sand barrier to prevent movement of fines from the soil into the porespaces of the gravel so that reduction in permeability is prevented.

Where the subsurface medium through which groundwater is to be directedis bedrock, suitable upgradient and downgradient zones can be producedby blasting the bedrock under conditions effective to produce upgradientand downgradient zones of fractured rock. Zones of fractured rock(commonly referred to as "blasted-bedrock zones" or "shatter zones") arepreferably formed by subsurface blasting using explosive charges withinmultiple shotholes drilled several feet apart from one another andoriented in the desired shape of the upgradient or downgradient zone.The shotholes used to fracture the rock are typically spaced along theareas to be blasted in a staggered fashion so as to create a broad zoneof shattering. Typically, the explosive charges are separated verticallyfrom each other within each shothole using stemming stone or othersuitable material, and the charges are detonated using milliseconddelays to reduce vibrations and ground heave to acceptable levels.

Methods for providing the treatment zones are varied and depend on thenature of the subsurface medium. For example, where the pre-existingsubsurface medium was soil, the treatment zone can be constructed byremoving soil from the treatment-zone area, such as by excavation ordrilling a hole or series of holes, and filling the treatment zone witha material having a permeability substantially greater than that ofsoil, such as sand, gravel, broken rock, zero-valent iron or other metalparticles, or combinations of these materials. It should be noted that,as used herein, drilling includes any process by which holes havingcircular or substantially circular cross-sections are produced,including, for example, augering. Injection wells or piezometers can beinstalled within the treatment zone's fill material, as appropriate, forsubsequent injection or placement of treating agents.

Alternatively, the treatment zone can be constructed by drilling a largehole or series of smaller holes at a contact between upgradient anddowngradient zones and installing a well-screen and riser in the drilledhole or in each of the series of drilled holes.

Yet another process for constructing the treatment zone involvesremoving soil from the treatment-zone area, such as by excavating thesoil, and installing permeable, preferably planar walls effective tosupport the soil surrounding the treatment zone. Suitable permeablewalls include those made of permeable sheet piling.

Where the subsurface medium is rock, the treatment zone can be providedby blasting the rock under conditions effective to fracture the rock inthe treatment-zone area. Injection wells or piezometers can be installedwithin the treatment zone's fractured rock, as appropriate, forsubsequent injection or placement of treating agents. Alternatively, thetreatment zone can be constructed by drilling the rock to produce a holeor a series of holes in the treatment zone. Typically, a single largehole can be drilled, or a series of smaller holes can be used. Thedrilled holes can be filled with a material having a permeabilitygreater than that of the surrounding rock, such as broken rock, gravel,or zero-valent iron or other metal particles. Alternatively, especiallyin the case where the surrounding subsurface medium is intact and stableand where the treatment method does not require filling the holes with apermeable material, the drilled holes can be left empty.

The method of the present invention is particularly well suited for thein situ treatment of groundwater containing one or more contaminants.Examples of contaminants which can be treated using the methods of thepresent invention include: toxic or hazardous chemical substances, suchas dissolved halogenated organic compounds, pesticides, hydrocarbons,heavy metals, cyanides, nitrates, radionuclides, or combinations ofthese; undesirable physical substances, such as colloids; and dangerousor potentially dangerous biological substances, such as pathogenicbacteria, parasites, or viruses.

Treatment of the subsurface groundwater takes place in situ, belowground, in the treatment zone to which the groundwater is directed bythe upgradient zone and directed out of by the downgradient zone.Treatment is carried out by contacting the one or more contaminants witha treating agent under conditions effective to treat at least one of theone or more contaminants present in the subsurface groundwater.

A number of treating agents and technologies can be used to treatgroundwater in the treatment zone. The particular technology used fortreatment depends on numerous factors, such as the geology, the typesand concentrations of the contaminants, the size of the contaminantplumes, the velocity of contaminant migration, and the physical,chemical, biological, legal, regulatory, political, and socialconstraints that may exist relative to treatment-zone design andimplementation.

Contacting the contaminants with the treating agent can be effected byinjecting a vapor-, liquid- or solid-phase treating agent directly intothe treatment zone. The treating agent can be carried to the treatmentzone by any suitable injection vehicle, such as a number ofsmall-diameter wells or piezometers. Under certain site-specificcircumstances a number of treating agents can be injected into thetreatment zone to treat contaminated groundwater.

For example, oxygenators, such as air, oxygen, hydrogen peroxide, ozone,or solid substances which slowly release oxygen over a period of time,can be used to oxygenate the groundwater and facilitate aerobicbiodegradation of the contaminant or contaminants, to the extent towhich they can be biodegraded under aerobic conditions. Oxygenators areparticularly useful to treat simple, non-chlorinated hydrocarbons andother organic substances amenable to ordinary aerobic biodegradation.

Alternatively, air or oxygen can be used in combination with aco-metabolite (such as methane, propane, or ammonia) to oxygenate thewater and to facilitate aerobic biodegradation of the co-metabolite and,at the same time, co-metabolism of the contaminant. This combination isparticularly useful to treat halogenated organic compounds. In manyinstances, use of a co-metabolite requires specific safety precautions(particularly in regard to concentrations being far below the lowerexplosive limits ("LELs")).

The treating agent can also be an electron acceptor other than oxygen,such as nitrate, particularly in cases where the contaminants aresusceptible to biodegradation under anoxic or anaerobic conditions.

Injectable reductants, such as sodium dithionate, starved reductive-ironbacteria, or colloidal iron, can also be used to create conditionsconducive to treating contaminated groundwater. In some cases, thereductant converts naturally occurring Fe(III) in minerals to Fe(II)which in turn facilitates degradation of, for example, organiccontaminants. In other cases the reductant can reduce inorganiccontaminants, such as hexavalent chromium, and facilitate theirprecipitation. In particular, the use of colloidal iron to treatcontaminated water is described in U.S. Pat. No. 5,266,213 to Gillham("Gillham"), which is hereby incorporated by reference.

Injectable oxidants, such as potassium permanganate or ozone, can alsobe used to directly attack and break down a variety of contaminants,including halogentated hydrocarbon compounds and other organiccontaminants.

As one skilled in the art will note, in some cases, the use ofinjectable treating agents would require specific safety precautions aswell as regulatory approval. As one skilled in the art would alsorecognize, not all contaminants can be treated with all of theabove-described treatment methods.

Alternatively to injecting the treating agent, the treating agent can beplaced in the holes or cavities formed in the treatment zone. Frequentlythis requires the construction of a subsurface cavity, such as a rockcavity or soil excavation, to house the treating agent. The subsurfacecavity can be a hole or cavity constructed at the treatment zone,constructed, for example, by the processes described above.

A number of promising treatment processes can be used in a rock cavityor soil excavation. One method, particularly well-suited for thetreatment of groundwater containing halogenated organic compounds("HOCs"), such as perchloroethene ("PCE"), trichloroethene ("TCE"),dichloroethene ("DCE"), and trichloroethane ("TCA"), is described inGillham, which is hereby incorporated by reference. Briefly, the methodinvolves contacting the HOCs with zero-valent iron or other zero-valentmetals placed below the water table. The metals preferably have a highspecific surface area, such as when the metal is in the form of filings,particles, or fibers. In the case where zero-valent iron is used, if,some years after the initial metal emplacement, it is discovered thatthere is loss of permeability caused by chemical precipitation in theupgradient iron, an electromagnet can be used to remove all or some ofthe iron. The iron can then be reactivated, for example, by acid washingor redistribution, and then re-emplaced. This treatment method can alsobe effective for groundwater containing excessive concentrations ofnitrate.

Treatment of groundwater containing HOCs or volatile organic compounds("VOCs"), such as benzene, toluene, ethylbenzene, and xylenes,(collectively referred to as "BTEX"), can be effected by sparging theHOCs or VOCs from the groundwater and collecting the sparged vapors in,for example, the vadose zone above the groundwater. This method involvescontacting the contaminated groundwater with a gaseous treating agent,such as compressed air or some other innocuous stripping gas. Thegaseous treating agent can be delivered down pipes installed to the baseof a treatment zone and then forced through microporous bubblers, suchas sparging tubes, to create small bubbles. Movement of the bubbles upthrough the contaminated groundwater in the treatment zone strip out theVOCs, and the VOC-rich stripping gas is then collected, for example, byvapor extraction, and removed for subsequent separation or treatment.

Yet another method for treating contaminated groundwater, particularlywell-suited for removing contaminants that tend to sorb onto solids orpartition into organic matter, involves contacting the contaminatedgroundwater with a sorptive material. Suitable sorptive materialsinclude activated carbon, resins, or polymers. Preferably the sorptivematerial is removable or regenerable.

Numerous other in situ methods can be used to treat the groundwaterdirected to the treatment zone. These include, for example, metal orradionuclide precipitation by, for example, pH adjustment;electrochemical methods to cause chemical changes to the groundwatercontaminants or to precipitate metals, preferably on an electrode;large-scale ultrasonic cavitation; and ultraviolet light treatment,optionally used in conjunction with hydrogen peroxide or ozonetreatment.

The present invention also relates to a method for directing groundwaterflowing in a general flow direction through a subsurface medium around aparticular subsurface location. The method includes providing anelongate permeable upgradient zone and an elongate permeabledowngradient zone, each of which has a major axis parallel to a non-zerocomponent of the general flow direction. The upgradient zone is locatedhydraulically upgradient from the particular location. That is, theupgradient zone is located so as to intercept groundwater which isflowing toward the particular location or which, in the absence of theupgradient and downgradient zones, would flow into the particularlocation. The downgradient zone is located hydraulically downgradientfrom and is in hydraulic communication with the upgradient zone. Thedowngradient zone is located downgradient from the particular location,so that groundwater flowing out of the downgradient zone does not flowinto the particular location. Groundwater is allowed to move from thesubsurface medium surrounding the upgradient zone into and through theupgradient zone. The groundwater then moves from the upgradient zone to,through, and out of the downgradient zone into the subsurface mediumsurrounding the downgradient zone. Each of the upgradient anddowngradient zones is situated within the subsurface medium and has apermeability substantially greater than the surrounding subsurfacemedium's permeability.

Suitable orientations for each of the upgradient and downgradient zonesinclude those described above in connection with the method of treatinggroundwater.

The method of the present invention for directing groundwater around aparticular location can also be practiced using several upgradient anddowngradient zones. In this embodiment the method further includesproviding one or more additional elongated upgradient zones and one ormore additional elongated downgradient zones. Each additional upgradientzone and downgradient zone has a permeability which is substantiallygreater that the permeability of the subsurface medium and each has amajor axis parallel to a non-zero component of the flow direction.Preferably, the number of additional upgradient zones is equal to thenumber of additional downgradient zones so that the total number ofupgradient zones is equal to the total number of downgradient zones. Theorientation of the additional upgradient and downgradient zones is notcritical to the practice of the present invention. However, it ispreferred that the downgradient zone and the additional downgradientzones be mirror images of the upgradient zone and the additionalupgradient zones reflected in a plane perpendicular to general flowdirection and passing through the particular location. Optimally, thetotal number of upgradient zones is 2, and the total number ofdowngradient zones is likewise 2. In a more preferred configuration, thedowngradient zone and additional downgradient zone are mirror images ofthe upgradient zone and the additional upgradient zone reflected in aplane perpendicular to ground surface, parallel to the general flowdirection, and passing through the particular location. Illustrativeconfigurations suitable for the practice of the present invention aredepicted in FIGS. 3A, 3B, and 3C. Flow lines 10 show the pathgroundwater having a flow direction 8 takes through subsurface medium 4around particular location 20 under the influence of upgradient zone 2and downgradient zone 12.

The method of the present invention for directing groundwater around aparticular location can be used in a variety of situations. For example,where the groundwater contains a contaminant, and the particularlocation is relatively free of the contaminant, the method could be usedto reduce migration of the contaminated groundwater through theparticular location thereby reducing the extent to which the particularlocation becomes contaminated by the contaminant. Where a particularlocation contains the contaminant in the subsurface medium and where thegroundwater is relatively free of the contaminant, the method of thepresent invention can be used to reduce the flow of uncontaminatedgroundwater through the contaminated particular location, therebyreducing the extent to which the uncontaminated groundwater becomescontaminated by the contaminant.

In addition, the groundwater velocity in the particular location wouldbe lowered greatly, by as much as an order of magnitude or more.Reducing flow velocity through a contaminated particular location wouldhave two potential advantages. First, slowing the movement of theimpacted groundwater would decrease the rate of contaminant plume growthand development, which could potentially be important since thedifficulty and cost of remediating contaminated groundwater is generallyan increasing function of plume length, breadth, and depth.

Second, decreasing the velocity of the contaminant would increase itsresidence time within a treatment center installed downgradient from theparticular region. This would allow for the deployment of much lessremedial agent (such as iron filings for reductive dechlorination, oroxygen injected for oxidative biodegradation) per unit time. Thedowngradient in situ treatment center can employ conventional treatmentprocesses, such as a funnel-and-gate treatment process (RemediationReview, 8(1): 6-7 (1995), which is hereby incorporated by reference) (inthe case of soil contamination), or the treatment method of the presentinvention. In either case, decreasing flow velocity through thecontaminated particular region would have the advantage of requiringless space in the treatment center, since less treating agent would berequired.

Optimization of the methods of the present invention involves a numberof factors. These include: the permeability of the upgradient anddowngradient zones, shape of the upgradient and downgradient zones,including their width and length, the permeability of the subsurfacemedium, the orientation of the upgradient and downgradient zones withrespect to each other and with respect to the general flow direction ofthe groundwater, and the size and shape of the treatment zone. Theinterrelationship of these factors is not fully understood. However, thefollowing is a brief description of how the invention is believed tooperate. This brief description in not meant to limit or otherwiseeffect the scope of the present invention but is only provided to guideoptimization of the invention to a particular situation. Accordingly,the scope of the present invention is not to be construed as beinglimited by the description which follows.

It is believed that the changes in groundwater flow direction at theupgradient and downgradient zones and the convergence of groundwaterflows to a treatment center are accomplished through engineeredgroundwater flow refraction. When groundwater flows out from one mediumand into a second medium having a higher permeability, the streamlinesin that water tend to be refracted, or reoriented, in differentdirections than the original streamline directions. The process issimilar but not identical to the refraction of light at the junctionbetween two media of differing refractive indices. One difference isthat the refraction of light is expressed by a sine law, whereas therefraction of groundwater, under idealized circumstances, is expressedby a tangent law. Refraction in actual groundwater systems, however,depends on three factors, only the first two of which are implied by thetangent law of refraction: (1) the orientation of the contact betweenthe two different media relative to the mean direction of groundwaterflow, (2) the relative permeability difference between the two media,and (3) the geometry of the two media and their contact surface. Theeffects of the last factor on refracted flow must generally be addressedthrough numerical flow modeling.

The basic concept of flow refraction can be understood in terms ofgroundwater flow through two subsurface porous media having a verticallyplanar contact that, in plan view, is oblique, but not perpendicular, tothe mean flow direction (See FIG. 4). The permeabilities of the twomedia differ, and the ratio of the permeabilities is one of theimportant variables governing flow refraction. This dimensionless ratiohas the same numerical value as the ratio of the hydraulicconductivities, where the term "hydraulic conductivity" accounts forfluid parameters affecting flow, such as viscosity and density, as wellas for the inherent transmissive capacity of the porous medium(Domenico, which is hereby incorporated by reference). In the commonexpression of the principles of flow refraction, the ratio of thehydraulic conductivities is used; this practice is followed here.

At the contact between the two media, the streamlines of the groundwaterflowing from one medium (with hydraulic conductivity K₁) to the contactform an angle θ₁ with the normals (or perpendiculars) to the contact,and the streamlines of the groundwater flowing from the contact into amedium of hydraulic conductivity K₂ form an angle θ₂ to these normals,all of which can be expressed according to the tangent law of refraction(Freeze, which is hereby incorporated by reference)

    tan(θ.sub.1)/tan(θ.sub.2)=K.sub.1 /K.sub.2     (Eqtn. 1)

In qualitative terms, Equation 1 states that groundwater flowing from alow-permeability medium into a very long, high-permeability mediumacross an oblique contact will tend to be refracted nearly parallel tothat contact if the high-to-low permeability ratio is very high(e.g., >100). If, for instance, a very long, narrow cell of sand wereplaced 45 degrees oblique to groundwater flow in a silt, where the K₂/K₁ ratio was 100, the angle of refraction with respect to the contactnormal would be

    θ.sub.2 =arctan[tan(θ.sub.1)K.sub.2 /K.sub.1 ]=arctan[tan(45)100]=89.4°                         (Eqtn. 2)

which means that flow would be nearly parallel to the contact.

If the high-permeability cell were of limited size, however, flow towardthe most downgradient end of the cell could not continue to be refractedas it would be along the upgradient portion of the cell, a concept thatis not apparent from inspection of Equation 1. This equation assumes aset of two semi-infinite media in which there is no tendency forhydraulic-head buildup at any point. If refraction were to continue allalong the length of the finite-length cell, then groundwater would becompressed at the downgradient end of the cell where thehigh-permeability medium meets the low-permeability medium. Flow in theinitial direction of refraction would thus be impeded. Thelow-permeability "wall" would tend to cause the hydraulic head to buildup and stop the refraction process some distance away from the wall.That flow would locally depart from the tangent law of refraction nearthe end of the cell under conditions in which the high-permeability zoneis limited in its spatial extent has been demonstrated as part of thiswork, based on a USGS MODFLOW groundwater flow representation.

However, groundwater flow can be made to converge to a locus if there isan associated downgradient region of dispersal having an equal capacityto disperse flow as the upgradient region has to collect flow. This isillustrated conceptually in FIG. 2D, where flow is converging toward a"flow-through" treatment zone through the two upgradient arms of anX-shaped refractive-flow system. The upgradient flow can converge inthis zone because the two symmetrical downgradient arms of therefractive-flow system carry groundwater away from the treatment zone asfast as the upgradient arms bring it in. In FIG. 5 (output from aMODFLOW model), the lines of equal hydraulic head, which areperpendicular to local groundwater flow, are illustrated in detail foran X-shaped refractive flow system where the system permeability is 100times that of the surrounding medium region. Representative groundwaterstreamlines 26 are also illustrated. FIG. 5 shows that groundwater flowis refracted from initial general flow direction 28 to a directionnearly parallel to the upgradient zones. The groundwater flow is thendirected through a permeable treatment zone at the center of the X andinto downgradient zones. The groundwater is conveyed out of thedowngradient zones to resume its general downgradient course. Allgroundwater flow contacting the upgradient zone passes through thepermeable treatment zone. The X-shaped refractive-flow system has beenshown through computer modeling to be highly efficient for directinggroundwater through a known, central location, where an in-situtreatment zone can be installed.

If a need were to exist for multiple treatment centers to be placedalong the upgradient arms of a refractive-flow system, it would benecessary to be able to estimate the spacing between the treatmentcenters. This spacing, interpreted in the equations as beingperpendicular to the mean direction of groundwater flow, can becalculated with reference to FIG. 6. S, the spacing between treatmentzones, is equal to T, the thickness of the high-permeability subsurfaceconduit (i.e. the thickness of the upgradient zones), multiplied by cos(α₁), the cosine of the angle between the major axis of the upgradientzone and the the mean (i.e., general) groundwater flow direction,multiplied by the term [(K₂ /K₁)-1].

    S=T[cos(α.sub.1)][(K.sub.2 /K.sub.1)-1]              (Eqtn. 3)

FIG. 6 shows how the ratio (S/T) varies with (K₂ /K₁) for differentrepresentative values of α₁. The values of T and K₂ can be estimatedfrom prior experience and, if necessary, can be confirmed, for example,by exploratory drilling and testing in the field after several shotholeshave been blasted. Adjustments in blasting methods can be made in thefield as necessary to achieve appropriate values for T and K₂. Thecalculations should give reasonable estimates of (S/T) forrefractive-flow systems with proper downgradient flow dispersal. Areasonable factor of safety should be applied in the design to accountfor subsurface heterogeneities and other unknowns.

Although the invention has been described for the purpose ofillustration, it is understood that such detail is solely for thatpurpose and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed:
 1. A method for directing groundwater flowing in ageneral flow direction through a subsurface medium around a particularsubsurface location comprising:providing an elongate permeableupgradient zone located hydraulically upgradient from the particularlocation and having a major axis parallel to a non-zero component of thegeneral flow direction; providing an elongate permeable downgradientzone located hydraulically downgradient from and in hydrauliccommunication with the upgradient zone, located downgradient from theparticular location, and having a major axis parallel to a non-zerocomponent of the general flow direction; and allowing groundwater tomove from the subsurface medium surrounding the upgradient zone into andthrough the upgradient zone to, through, and out of the downgradientzone into the subsurface medium surrounding the downgradient zone,wherein each of the upgradient zone and downgradient zone is situatedwithin the subsurface medium and has a permeability substantiallygreater than the surrounding subsurface medium's permeability.
 2. Amethod according to claim 1, wherein the subsurface medium is soil.
 3. Amethod according to claim 2, wherein the upgradient zone and thedowngradient zone each independently comprises a material selected fromthe group consisting of sand, gravel, broken rock, and combinationsthereof.
 4. A method according to claim 3, wherein said providing theupgradient zone and said providing the downgradient zonecomprises:removing soil from the upgradient zone and the downgradientzone and filling the upgradient zone and the downgradient zone with amaterial selected from the group consisting of sand, gravel, brokenrock, and combinations thereof.
 5. A method according to claim 1,wherein the subsurface medium is rock.
 6. A method according to claim 4,wherein each of the upgradient zone and the downgradient zone comprisesfractured rock.
 7. A method according to claim 6, wherein said providingthe upgradient zone and said providing the downgradient zonecomprises:blasting the rock under conditions effective to fracture therock in the upgradient zone and the downgradient zone.
 8. A methodaccording to claim 1, wherein the groundwater comprises a contaminantand wherein the particular location is relatively free of thecontaminant.
 9. A method according to claim 8, wherein the contaminantis selected from the group consisting of toxic materials, hazardousmaterials, pathogens, malodorous materials, noxious materials, andcombinations thereof.
 10. A method according to claim 1, wherein theparticular location comprises a contaminant and the groundwater isrelatively free of the contaminant.
 11. A method according to claim 10,wherein the contaminant is selected from the group consisting of toxicmaterials, hazardous materials, pathogens, malodorous materials, noxiousmaterials, and combinations thereof.
 12. A method according to claim 1,wherein the general flow direction forms an angle of α with respect tothe upgradient zone's major axis and wherein α is from about 30° toabout 60°.
 13. A method according to claim 12, wherein the general flowdirection forms an angle of 180-α° with respect to the downgradientzone's major axis and wherein an angle of 180-2α° exists between thedowngradient zone's major axis and the upgradient zone's major axis. 14.A method according to claim 1, wherein the upgradient zone has asubstantially uniform width, the downgradient zone has a substantiallyuniform width, and the widths and lengths of the upgradient anddowngradient zones are substantially the same.
 15. A method according toclaim 1, further comprising:providing one or more additional elongateupgradient zones, each located hydraulically upgradient from theparticular location and each having a major axis parallel to a non-zerocomponent of the general flow direction and providing one or moreadditional elongate permeable downgradient zones each locatedhydraulically downgradient from and in hydraulic communication with atleast one of the additional upgradient zones, each located downgradientfrom the particular location, and each having a major axis parallel to anon-zero component of the general flow direction, wherein each of theadditional upgradient zones and each of the additional downgradientzones is situated within the subsurface medium and has a permeabilitysubstantially greater than the surrounding subsurface medium'spermeability.
 16. A method according to claim 15, wherein the number ofadditional upgradient zones is equal to the number of additionaldowngradient zones.
 17. A method according to claim 15, wherein thedowngradient zone and the additional downgradient zones are mirrorimages of the upgradient zone and the additional upgradient zones,reflected in a plane perpendicular to general flow direction and passingthrough the particular location.
 18. A method according to claim 17,wherein the number of additional upgradient zones is one and the numberof additional downgradient zones is one and wherein the downgradientzone and additional downgradient zone are mirror images of theupgradient zone and the additional upgradient zone, reflected in a planeperpendicular to ground surface, parallel to the general flow direction,and passing through the particular location.